Generally, when a mobile cellular telephone unit moves around within the coverage area of the cellular telephone system, it will exit the effective coverage area of one base station(s) and enter the effective coverage area of another base station(s). (A base station is also sometimes referred to as a cell-site. The base stations in a cellular telephone system are used to control the communications links from the mobile cellular telephone units and to provide access to the public-switched telephone network.) To provide uninterrupted service, the control of the mobile cellular telephone unit must be passed from base station(s) with the effective coverage area that the mobile cellular telephone unit is exiting and to the base station(s) with the effective coverage area that the mobile cellular telephone unit is entering. This transfer of control is commonly referred to as a hand-off.
There are typically two different types of hand-offs, a hard hand-off and a soft hand-off. Hard hand-offs involve the termination of the control via the base station(s) from the exited effective coverage area prior to the establishment of the control via the base station(s) with the entered effective coverage area. Hard hand-offs can result in the unexpected termination of an existing communications link with the mobile cellular telephone unit if the control with the base station(s) of the entered effective coverage area is not established in a timely manner. Alternatively, soft hand-offs permit a mobile cellular telephone unit to connect to multiple base stations. By simultaneously connecting to multiple base stations, the quality of the communications link is improved. Additionally, since the mobile cellular telephone unit is simultaneously connected to multiple base stations, it is possible for one or more of these base stations to drop off as the mobile cellular telephone unit moves without disconnecting the communications link.
In a code-division multiple access (CDMA) cellular telephone system, both hard and soft hand-offs are permitted. Both soft and hard hand-offs are controlled by a neighbor list. Each base station in the CDMA cellular telephone system has a neighbor list. The neighbor list of a base station is a list of pilot channels of base stations that the mobile cellular telephone unit may handoff to. The mobile cellular telephone unit is continually searching for potential handoff candidates. The searching algorithm focuses on the base stations defined in the neighbor list, in particular a candidate set in the neighbor list. However, the searching algorithm does search all possible pilot channels. Poor quality pilot channels are dropped automatically and good pilot channels are added (if permitted). Ideally, the neighbor list is kept small so that potential pilot channels can be scanned quickly.
The use of the neighbor list can help reduce the time required to search for pilot channels because the neighbor list effectively informs the mobile which pilot offsets should be searched first. To scan all possible pilot channels can take a considerable amount of time and the neighbor list prioritizes the search order for the mobile cellular telephone unit (potentially saving it a lot of time). Once the mobile cellular telephone unit finds a pilot channel (or a plurality of pilot channels) of sufficient strength, the mobile cellular telephone unit can initiate a soft hand-off. In a CDMA cellular telephone system, if a mobile cellular telephone unit is under the control of a plurality of base stations (when there are more than one control links established with different base stations), the quality of the communications channel tends to increase due to the use of signal combining techniques similar to multi-path combining.
When a mobile cellular telephone unit is communicating with a single base station and it requests hands off to another base station, this is commonly referred to as a one-way hand-off. When a hand-off involves two base stations requesting a third, it is commonly referred to as a two-way hand-off. In general, when a mobile cellular telephone unit is communicating with N base stations and requests an N+1 base station, it is performing an N-way hand-off. When a hand-off involves different sectors of a single base station, this is commonly referred to as a softer hand-off.
In a majority of cases, the neighbor list for each base station is created by having a scanner at various places within the base station's effective coverage area detecting as many pilot channels as it can. The scanner detects pilot channels by scanning for pseudo-random number (PN) sequences. Each base station's pilot channel (within a local area) should have a unique PN sequence, although system wide, a single PN sequence may be used by several different base stations. Then, all of the pilot channels are placed in the base station's neighbor list. This method of neighbor list creation ensures that every pilot channel that is detectable within a give base station's effective coverage area is in the neighbor list.
One disadvantage of the prior art is that by placing every detectable pilot channel in the neighbor list, the neighbor list becomes unnecessarily large. A neighbor list that is too large results in an increase in the amount of time that it takes for a mobile cellular telephone unit to search for a pilot channel. This can increase the chance of a communications link being disconnected if a new control link is not established prior to the mobile cellular telephone unit exits the effective coverage area of the base station(s) to which it is currently linked. A problem related to this is that by the time the mobile cellular telephone unit gets around to measuring a particular PN sequence that may or may not correspond to a pilot channel in the neighbor list, it may have become so strong that it has become an interferer and may cause the call to drop.
A second disadvantage of the prior art is that by placing every detectable pilot channel in the neighbor list, it is possible to have multiple pilot channels with the same PN sequence, but associated with different base stations in the neighbor list. This results in ambiguity as to from which base station the pilot channel is being transmitted. This confusion can prevent a hand-off from taking place and may result in the call to drop.
A third disadvantage of the prior art is that by having every detectable pilot channel in the neighbor list, the resulting neighbor list may exceed the neighbor list storage capacity of the mobile cellular telephone unit. This may result in the inclusion of poor hand-off candidates in a portion of the neighbor list stored in the mobile cellular telephone unit while good candidates are excluded due to lack of storage space.